A direct-type tire inflation pressure detector has been known. This type of tire inflation pressure detector uses a transmitter that is directly attached to a tire wheel of a vehicle. The transmitter has a sensor such as a pressure sensor. An antenna and a receiver are mounted on a body of the vehicle. When the transmitter transmits data including a detection signal from the sensor, the receiver receives the data via the antenna and detects a tire inflation pressure based on the data. The direct-type tire inflation pressure detector determines whether the data is transmitted from the vehicle equipped with the direct-type tire inflation pressure detector or another vehicle. Further, the direct-type tire inflation pressure detector determines which wheel is provided with the transmitter. For this purpose, each data transmitted from the transmitter contains ID information that discriminates between the vehicle and the other vehicle and identifies a wheel to which the transmitter is attached.
In order to locate the transmitter, the receiver needs to pre-register the ID information about each transmitter in association with each wheel position. If tire rotation is performed, the receiver needs to re-register the ID information. For example, patent document 1 proposes a method of automating this registration.
Specifically, in the method according to patent document 1, it is determines whether the wheel reaches a specified rotation position based on an acceleration detection signal from an acceleration sensor included in the transmitter attached to the wheel. The vehicle also detects a rotation position of the wheel based on a wireless signal from the transmitter. The vehicle monitors a change in a relative angle between the rotation positions to specify the wheel position. This method monitors a change in the relative angle between the wheel rotation position detected by the vehicle and the wheel rotation position detected by the wheel based on a deviation in a specified number of data. The method specifies the wheel position by determining that a variation with reference to an initial value exceeds an allowable value.
However, the method described in patent document 1 specifies the wheel position based on whether the variation falls within a variation allowable range defined by the allowable value. Therefore, the method cannot specify the wheel position while the variation falls within the variation allowable range. In addition, a certain large number of data is needed because the method specifies the wheel position based on the standard deviation. The method cannot specify the wheel position until the necessary number of data is acquired. Accordingly, it takes a long time to specify the wheel position.